1. Field of the Invention
The present invention relates to a frequency stabilizer for obtaining a stabilized frequency signal depending on predetermined frequency settings in the microwave and millimeter wave frequency bands, or the like.
2. Description of the Prior Art
It is well known for those skilled in the art to use a magnetic resonator in a high frequency oscillator. The Japanese Patent Laid-Open No. 140803/1989 (corresponding to the U.S. Pat. No. 4,887,052) discloses an example of this structure.
FIG. 1 illustrates the structure of a typical example of frequency synthesizers in the prior art. In FIG. 1, the frequency synthesizer comprises a magnetic resonator 1, a magnetic circuit 2 for applying a magnetic field to the magnetic resonator 1, a main coil 3 and a yoke 4 forming the magnetic circuit, an active element 5 connected to the resonator 1 and a matching/amplifying circuit 6. An oscillator is formed by the magnetic resonator 1, magnetic circuit 2, active element 5 and matching/amplifying circuit 6. The frequency synthesizer further includes a coupler 8 connected to the oscillator 7, a variable frequency divider 9 mutually connected between the terminal of coupler 8 and the terminal of the coil 3, a phase detector 10, a reference signal generator 11, a loop filter 12, a coil driving circuit 13 and a feedback circuit 16 connected to the active element 5.
Moreover, the coupler 8 is connected to the oscillation output terminal 14, while the variable frequency divider 9 is connected to the frequency setting input terminal 16.
Next, the operation of the frequency synthesizer will be explained. The magnetic resonator 1 has a resonance frequency in a microwave or millimeter wave frequency band which is determined by the magnetic field generated in the magnetic circuit 2. In this case, when the structure of the magnetic resonator 1 and circuit constants of the feedback circuit 16 and matching/amplifying circuit 6 are selected so that the product .vertline..GAMMA.1.vertline...vertline..beta.2.vertline. of the absolute values of the complex reflection coefficients .GAMMA.1 and .GAMMA.2 is equal to or greater than one and the sum .angle..GAMMA.1+.angle..GAMMA.2 of phase angles .angle..GAMMA.1 and .angle..GAMMA.2 is zero, where .GAMMA.1 is the reflection coefficient of the magnetic resonator 1 observed from the active element 5 such as a transistor and .GAMMA.2 is the complex reflection coefficient of the active element 5 observed from the magnetic resonator 1, an electromagnetic wave is amplified each time it passes forward and backward between the active element 5 and magnetic resonator 1 and the amplification is continued until the active element 5 is saturated. As a result, oscillation is established. This oscillating output can be extracted from the oscillation output terminal 14 through the matching/amplifying circuit 6 and coupler 8.
The frequency of this oscillation output is determined by the magnetic field generated by the current flowing through the coil 3 of the magnetic circuit 2.
When the frequency division ratio of the variable frequency divider is set to "n" through the frequency setting input terminal 15, the frequency f.sub.1 of the oscillation output signal extracted by the coupler 8 is divided into f.sub.1 /n and this frequency-divided signal and the fixed frequency f.sub.2 obtained from the reference signal generator 11 utilizing a crystal oscillator as the oscillation source are put to the phase detector 10, a voltage corresponding to f.sub.1 /n-f.sub.2 can be obtained as an output. As shown in FIG. 2, variations in voltage with time include short period changes caused by noise and long period changes with temperature which are superimposed on each other. In general, this output voltage of the phase detector is applied to the loop filter 12 operating as a low-frequency band-pass filter in order to extract a voltage signal having periodic components of about several kHz or lower and this voltage is then reduced to 0 V by controlling the current flowing through the coil driving circuit 13. As a result, the fluctuation of the oscillating frequency of about several kHz or less can be controlled and stabilized as shown in FIG. 3. In this case, frequency spectrum characteristics near the oscillating frequency are shown in FIG. 4, where spectra, that is, phase noises corresponding to the frequency fluctuation components indicated by a broken line are suppressed. Moreover, since such control is carried out so that f.sub.1 /n-f.sub.2 =0 can be obtained, the oscillating frequency f.sub.1 can be varied by changing the frequency division ratios "n" applied through the frequency setting input terminal 15.
Since the frequency synthesizer in the prior arts is arranged as explained heretofore, any attempt to obtain a pure signal by controlling the fluctuation of the oscillating frequency having shorter periods as shown in FIG. 5 results in a problem that the resistance offered to the flow of a coil current of high frequency becomes strong due to the self-inductance of the coil 3 even when the upper limit of the pass-band frequency of the loop filter 12 is made higher. Moreover, since the coil current lags the output voltage of the loop filter 12, control of fluctuation of the oscillating frequency is difficult. Moreover, when self-inductance is lowered, for example, by reducing the number of turns of coil 3, there arises a problem that a large coil current is required for obtaining the predetermined frequency control range and a current amplifying transistor having a large current capacity is required for the coil driving circuit 13, resulting in an increase in cost. In addition, there is a problem that heat generation by the coil 3 increases and the change in temperature of the magnetic resonator 1 and magnetic circuit 2 will become larger than that in the ambient temperature.